1. Field of the Invention
This invention relates to seismic exploration, and, more particularly, to a seismic array with bidirectional capabilities and a method of seismic exploration.
Description of the Prior Art
In seismic exploration, seismic waves are commonly used to probe the earth's crust as a means of determining the type and location of subsurface formations. The earth's crust can be considered a transmission medium or filter whose characteristics are to be determined by passing seismic waves through that medium. In the reflection seismic method seismic waves or impulses are generated at a point at or near the earth's surface, and the compressional mode of these waves is reflected from subsurface acoustic impedance boundaries and detected by arrays of seismic detectors located at the surface. The seismic detectors convert the received waves into electrical signals which are sensed and recorded in a form which permits analysis. Skilled interpreters can discern from such an analysis the shape and depth of subsurface reflection boundaries and the likelihood of finding an accumulation of minerals, such as oil and gas.
It is well-known to those skilled in the art that the complex of vibrations received at a given seismic detector array do not consist totally of waves reflected from subsurface boundaries. Rather, the array also detects unwanted random seismic events, as well as various high amplitude modes of spatially-coherent source-generated seismic events whose principal direction of propagation is horizontal, i.e., along and near the free surface. It is essential that the effects of these unwanted horizontally-propagated waves be reduced by utilizing the principles of a directional antenna to attenuate the magnitude of the electrical signal produced by the geophones in response to these waves.
In U.S. Pat. No. 2,698,927 to Parr, there is disclosed a method of reducing the effects of the coherent horizontally-propagated seismic waves. This method comprises assigning relative sensitivity values to geophones in an array. The sensitivity values are selected according to recognized antenna theory so as to reduce the magnitude of the electrical signal produced in response to the unwanted spatially-coherent seismic waves. Parr refers to his method as a "tapered sensitivity" method, since the sensitivity of the sensing devices in a given array is reduced toward either end of the array from a central point when the transducers are aligned radial to the energy sources used to generate the seismic signal. A combination of the spacing of the individual sensors, the length of the geophone array, and the wavelength bandwidths of the interference to be attenuated comprise the criteria for assigning the relative sensitivity to be employed at each geophone.
A good general review of the weighting of seismometer arrays is given by Parr and Mayne in Geophysics, Vol. 20, pages 539-564 (1955), and Holzman, in Geophysics, Vol. 28 (1963), discloses that the optimum attentuation of the effects of coherent horizontally-propagated seismic waves may be achieved by applying Chebychev weighting coefficients to the sensors in an array. The combined teachings of Parr, Parr and Mayne and Holzman are recognized standards for reducing the effects of the unwanted vibrational energy.
There have, however, been other proposals for reducing the amplitudes of the horizontally-propagated energy which is recorded. For example, in U.S. Pat. No. 2,747,172 to Bayhi, two methods are disclosed for obtaining a tapered geophone array that is designed to have a response which attenuates the electrical signals produced in response to the unwanted vibrational energy. The first method involves constructing an array having a plurality of geophones at each location in the array. The number of geophones is maximum at the center point of the array and tapers off in the direction of the ends of the array.
The second method disclosed by Bayhi is to use a single geophone at each location of the array and to install a voltage divider network across each geophone in the array. The voltage divider network at each geophone consists of resistors, and the values of the resistors used are chosen so that the geophone in the physical center of the array has the greatest sensitivity, while the geophones at the end of the array have the least sensitivity. The weighted geophone array Bayhi is apparently not bidirectional, and it appears that difficulty in maintaining a substantially constant damping factor between all geophones in the array will be encountered with the array of Bayhi.
Later, in U.S. Pat. No. 3,096,846 to Savit, there is disclosed a method of determining the seismometer weights to be applied in array tapering by using a moveout criterion. The results of Savit's method is that the distance between individual seismic detectors in a given array may not be uniform and the sensitivity of the individual seismic detectors will vary according to the moveout criterion.
From practical considerations it has been found expedient to approximate a desired weighting by constructing an array having a plurality of seismic detectors at each location in the array (e.g., as taught by Bayhi), with the number of seismic detectors at each location dictating the weighting coefficient of that location. Since it is generally agreed that the Chebychev coefficients are the optimal weights and since these coefficients are not integral numbers, the actual number of individual seismic detectors that would be required to implement (even approximately) these coefficients is very large. Hence, for this practical reason, Chebychev-weighted arrays have not generally been attempted nor realized.
Two recent patents disclose apparatus for applying Chebychev weighting coefficients to the seismic detectors in an array. In U.S. Pat. No. 3,863,200 to Miller, there is disclosed a built-in seismometer amplifier which permits the sensitivity of the individual seismometer to be adjusted at a given location. It will be noted from the Miller patent that a separate pair of wires is required to convey the signal generated at each seismometer back to a suitable recording point. Consequently, a multi-pair cable is required between the array of seismometers and the recording point in order to utilize the build-in seismometer amplifier that Miller discloses.
In U.S. Pat. No. 3,863,201 to Briggs there is disclosed a seismometer weighting apparatus to apply weighting coefficients to individual seismometer signals at a recording point. Briggs states that the apparatus may be utilized with a uniformly weighted and uniformly spaced array. It will be noted from the Briggs patent, however, that a multi-pair cable is required between the recording point and each detector in the array.
No seismic detector array has been developed to date which provides weighted sensitivity at the individual seismic detectors in the array, which maintains essentially constant damping between seismometer units, which substantially reduces the number of seismic detectors to achieve weighted sensitivity, which has bi-directional capabilities, and which supplies data from all seismic detectors to a given end of the array over a single signal-carrying medium. This useful and novel result has been achieved with the improved seismic array of the present invention.